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Dorsal not erect enough

A few weeks ago a breeder brought me a pair of kois in a bag. At the time I saw nothing wrong, but when I got them into the tank I thought the male was keeping his dorsal fin down because he was a little timid of his new surroundings. There were no other fish in the tank but him and his female partner, its a 2ft x 1ft by 15 high that I use for quarantine or sometimes breeding.

This male retains his dorsal fin in this position, which I would say is about 30 degrees to his body. I will try and get a photo soon. I cant tell if it is a deformity which should have been culled or if the fin would become more erect if he flexed it. Is this a genetic defect, or is it environmental due to inproper rearing conditions?

A few weeks ago a breeder brought me a pair of kois in a bag. At the time I saw nothing wrong, but when I got them into the tank I thought the male was keeping his dorsal fin down because he was a little timid of his new surroundings. There were no other fish in the tank but him and his female partner, its a 2ft x 1ft by 15 high that I use for quarantine or sometimes breeding.

This male retains his dorsal fin in this position, which I would say is about 30 degrees to his body. I will try and get a photo soon. I cant tell if it is a deformity which should have been culled or if the fin would become more erect if he flexed it. Is this a genetic defect, or is it environmental due to inproper rearing conditions?

Would like to see pictures. It can be a deformity but not necessarily hereditary.

If you want to identify its inheritability, the cleanest way to do it is to cross it with a silver angelfish:

1. If it isn't a recessive mutation (therefore a dominant undesirable trait) then all the resulting progeny will exhibit the deformity. That being the case, you'll need to decide what to do with the male.

2. If it doesn't express in the resulting F1's, then it is one of the following:

a. it is a recessive mutation - 100% of its progeny will carry it but not express it at this point since they only have 1 copy of the trait in their genetic makeup.
b. it isn't inheritable - 100% of its progeny will not carry it.

Unfortunately, if it happens to fall under number 2 the only way you can be sure is to do a sibling cross within the resulting F1's such that if it shows up in any of the offspring, you can be assured that the original male you used carries a hereditary recessive undesirable mutation and therefore inappropriate for further breeding. If no visible traces of the defect is the case then your male is a safe bet. I wish there was a better way to assess it but look at it this way, at least it can be determined (but unfortunately not without a good amount of time and effort invested on your part).